Vacuum ultraviolet absorption spectroscopy system and method

ABSTRACT

An efficient absorption spectroscopy system is provided. The spectroscopy system may be configured to measure solid, liquid or gaseous samples. Vacuum ultra-violet wavelengths may be utilized. Some of the disclosed techniques can be used for detecting the presence of trace concentrations of gaseous species. A preferable gas flow cell is disclosed. Some of the disclosed techniques may be used with a gas chromatography system so as to detect and identify species eluted from the column. Some of the disclosed techniques may be used in conjunction with an electrospray interface and a liquid chromatography system so as to detect and identify gas phase ions of macromolecules produced from solution. Some of the disclosed techniques may be used to characterize chemical reactions. Some of the disclosed techniques may be used in conjunction with an ultra short-path length sample cell to measure liquids.

This application is a continuation of pending U.S. patent applicationSer. No. 14/538,181, filed on Nov. 11, 2014 and entitled “VacuumUltraviolet Absorption Spectroscopy System And Method”, which is acontinuation of U.S. patent application Ser. No. 13/792,853, filed onMar. 11, 2013, now issued as U.S. Pat. No. 9,116,158, and entitled“Vacuum Ultraviolet Absorption Spectroscopy System And Method” whichclaims priority to Provisional Patent Application No. 61/715,432 filedOct. 18, 2012; the disclosures of which are expressly incorporatedherein by reference.

BACKGROUND OF THE INVENTION

The present disclosure relates to the field of absorption spectroscopy.More specifically, it provides a means by which vacuum ultraviolet (VUV)light may be employed to facilitate spectroscopy of matter in the VUVregion.

Vacuum ultraviolet (VUV) light is strongly absorbed by virtually allforms of matter. Hence, from a theoretical viewpoint VUV absorptionmight be expected to provide an ideal means of probing such.Unfortunately in practice, realizations of VUV based absorption systemshave remained largely elusive due to a lack of suitable (i.e. efficient)components and demanding environmental considerations. As a resultrelatively little effort has been directed towards exploiting thisregion of the electromagnetic spectrum.

It follows that there would be great benefit associated with overcomingthese difficulties and developing VUV absorption systems that could beused to investigate a wide range of materials. It would be furtheradvantageous if such systems could be readily coupled with establishedanalytical techniques so as to facilitate integration into existinglaboratories with minimum effort and expense.

SUMMARY OF THE INVENTION

The disclosure herein relates to the field of optical spectroscopy. Inone embodiment, a highly efficient absorption spectroscopy system foroperation in the VUV is provided. The spectroscopy system may bespecifically configured to measure solid, liquid or gaseous samples.

In one embodiment the disclosed techniques can be used as anon-destructive means of detecting the presence of trace concentrationsof gaseous species. In another embodiment the disclosed techniques maybe used in conjunction with a gas chromatography (GC) system so as todetect and identify species eluted from the column. In yet anotherembodiment the disclosed techniques may be used in conjunction with anelectrospray interface and a liquid chromatography (LC) system so as todetect and identify gas phase ions of macromolecules produced fromsolution. In yet a further embodiment the disclosed techniques may beused to characterize chemical reactions. In a further embodiment thedisclosed techniques may be used in conjunction with an ultra short-pathlength sample cell to measure liquids.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention and advantagesthereof may be acquired by referring to the following description takenin conjunction with the accompanying drawings, in which like referencenumbers indicate like features. It is to be noted, however, that theaccompanying drawings illustrate only exemplary embodiments of thedisclosed concept and are therefore not to be considered limiting of itsscope, for the disclosed concept may admit to other equally effectiveembodiments.

FIG. 1—Vacuum ultraviolet absorption detector with collimated beam flowcell for use in conjunction with gas chromatography system.

FIG. 2—Vacuum ultraviolet collimated beam absorption flow cell withthermal isolation couplings and associated fittings.

FIG. 3—Alternate embodiment vacuum ultraviolet absorption detector forgas chromatography system with dedicated flow cell chamber.

FIG. 4—Vacuum ultraviolet absorption detector with focused beam flowcell for gas chromatography system.

FIG. 5—Chromatogram resulting from a 0.1 μL split injection of a 3:5solution of ethanol and methylene chloride. The y-axis corresponds tothe absorbance of the solution averaged over the 125-220 nm wavelengthregion.

FIG. 6—Absorbance spectra from 125-200 nm corresponding to the maximumabsorbance scans for the EtOH and CH₂Cl₂ peaks in the chromatogram ofFIG. 5.

FIG. 7—Absorbance ratio generated by dividing the ethanol absorbance bythe methylene chloride absorbance on a wavelength-by-wavelength basis(top). Ethanol cross section generated by multiplying the absorbanceratio by the methylene chloride cross section on awavelength-by-wavelength basis (bottom).

FIG. 8—Chromatogram of unleaded gasoline obtained using 125 nm-220 nmspectral filter.

FIG. 9—Comparison of 125 nm-240 nm absorbance spectra for an aliphatichydrocarbon (top) and toluene (bottom). The response from aliphatichydrocarbons is generally concentrated in the far-VUV region, whereasfor many aromatic compounds the maximal response lies in the 180-190 nmregion.

FIG. 10—Chromatogram of unleaded gasoline obtained using 150 nm-200 nm(top) and 125 nm-160 nm (bottom) filters. The longer wavelength filter(top) suppresses the response of aliphatic hydrocarbons relative toaromatic compounds, while the shorter filter (bottom) enhances them.

FIG. 11—Chromatogram of unleaded gasoline obtained using 200 nm-220 nmfilter (top). The filter suppresses both aliphatic hydrocarbons andaromatics in favor of polycyclic aromatic hydrocarbons. Absorbancespectrum for the naphthalene peak (bottom) showing the maximal responsein the 200 nm-220 nm region.

FIG. 12—Chromatogram obtained using vacuum ultraviolet absorptiondetector. Peaks correspond to (I) methanol and (II) methylene chloride.

FIG. 13—Vacuum ultraviolet transmittance spectrum for methanol.

FIG. 14—Vacuum ultraviolet transmittance spectrum for methylenechloride.

FIG. 15—Vacuum ultraviolet absorption system with stand alone gas cell.

FIG. 16—Ultra short path length liquid flow cell for use in conjunctionwith VUV absorption detector.

FIG. 17—Cross sectional views of ultra short path length liquid flowcell indicating middle of measurement region (top), edge of measurementregion (middle), and edge of flow cell (bottom).

FIG. 18—Vacuum ultraviolet absorption detector with collimated beam flowcell for use in conjunction with high performance liquid chromatographysystem.

FIG. 19—Alternate embodiment vacuum ultraviolet absorption detector forhigh performance liquid chromatography system with focused beam flowcell.

FIG. 20—Vacuum ultraviolet gas absorption flow cell with integratedelectrospray interface for use with liquid samples.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The current disclosure presents a VUV absorption system which isparticularly well suited to investigating liquids and gases. In oneembodiment the system is configured as a non-destructive detector to beused in combination with a gas chromatography (GC) system. In anotherembodiment the system is configured as a detector to be used incombination with a liquid chromatography (LC) system for the study ofliquids. In yet another embodiment the system is configured to be usedin conjunction with both an LC system and an electrospray interface soas to enable the study of species that would otherwise be difficult torender in gaseous form.

In gas chromatography (GC), a sample is vaporized and transported alongwith an inert carrier gas (referred to as the mobile phase) into a tubecalled a column. The column contains a stationary phase that interactswith the various components of the sample. The interaction of the samplecomponents with the stationary phase causes them to elute from the endof the column at different times; with the result that the sample is“separated” into its constituent components. Eluted components aredetected by means of a detector.

There are many different types of GC detectors in existence; the twomost common are the flame ionization detector (FID) and the thermalconductivity detector (TCD). Both are sensitive to a considerable rangeof components, and both work well over a wide range of concentrations.While extensively employed for many applications both of these detectorsare regrettably found lacking with regards to selectivity. Most of theremaining GC detectors exhibit the inverse set of properties; that isthey are selective, but are only sensitive to a specific range ofcomponents. Currently the lone exception to this generalization is themass spectrometer (MS) detector. Mass spectrometers are sensitive to awide range of components and at the same time provide high selectivity.Unfortunately MS detectors are expensive and complicated instrumentsrequiring considerable expertise to operate. As such, it follows thatthere would be great benefit in the development of a sensitive andselective GC detector which is simple to operate and near universal inits response to a wide range of components.

A schematic representation of one embodiment of the current system ispresented in FIG. 1. As is evident in the figure the main elements ofthe system 100 include the source module 102, the flow cell 104, and thedetector module 106. In operation, light from the VUV source 110 isblocked or allowed to pass by a computer-controlled shutter mechanism112 and collimated by a first VUV optic 114 which then directs itthrough the flow cell. The shutter actuator is typically locatedexternal to the source module and connected via a vacuum feed-through inan effort to minimize contamination sources in the optical path of theinstrument.

While not explicitly shown in FIG. 1, it is noted that the source modulecould be equipped with appropriate beam reducing VUV optics to shrinkthe beam diameter (relative to that of the flow channel) in order toincrease the photon flux passing through the flow cell. In addition, thesource module could also be equipped with a photo detector that could beused to monitor the output of the source as a function of time. Such adetector may also prove useful in distinguishing changes in sourceoutput from those caused by contamination downstream in the opticalsystem.

Preferably the source generates a broad band spectral output; however inspecific applications intense line sources may also be desirable. Aparticularly well-suited source is a deuterium lamp equipped with a VUVtransparent window. Such windows are typically constructed of one of ahost of fluoride compounds (i.e. magnesium fluoride MgF₂, lithiumfluoride LiF, etc). The source is typically mounted in such a manner asto permit an air tight seal with the source module.

In one embodiment of the system the first VUV optic is a replicatedoff-axis toroidal mirror finished with an aluminum/MgF₂ coating toenhance VUV reflectivity. The surface roughness of the optic may be wellcontrolled to minimize scattering losses. In select instances lensescould be used in place of mirrors; however such an option may result inabsorption losses and chromatic aberrations.

The collimated light beam 116 exiting the source module passes throughanother VUV transparent window 118 as it enters the flow cell. Thewindow is mounted such that a leak tight seal separates the environmentsof the source module and the flow cell. The environment within thesource module is maintained via gas connections 120 depicted in thefigure such that the concentration of absorbing species (i.e. oxygen,water, etc) is low enough so as to not appreciably absorb the VUV photonflux. This may be accomplished using vacuum and/or purge techniques(with a largely non-absorbing gas like nitrogen, helium, hydrogen, etc).While not represented in the figure it is understood that theseconnections may also incorporate valves, regulators, controllers and thelike, as required to maintain the controlled environment. Furthermore,it may be desirable to introduce very low concentrations of certainspecies into the controlled environment to promote cleaning of opticalsurfaces and/or prevent the build up of contaminants on such.

The source module and the flow cell are connected via a thermalisolation coupling 122. Such couplings are typically constructed ofceramics exhibiting low thermal conductivity so as to permit heating ofthe flow cell without significantly affecting the temperature of therest of the system. The drawback with many ceramics, however, is thatthey are brittle and prone to fracture. An alternate approach is toconstruct the thermal standoffs using thin walled stainless steeltubing. While the thermal conductivity of stainless steel is much higherthan that of most ceramics, the cross sectional area of thin walledtubing can be very small, thus limiting the conductance to an acceptablelevel while still maintaining sound mechanical properties. The thermalisolation coupling is typically sealed to the flow cell and sourcemodule using metal and/or specialized high temperature, low out-gassingseals so as to minimize the release of contaminants which may degradeoptical performance.

A simplified schematic of a gas chromatograph 130 is also depicted inFIG. 1. A sample is vaporized and introduced to the carrier gas streamat an injector port 132. The carrier gas and sample enter the column133. The oven maintains an elevated temperature as the sample interactswith the column. The carrier gas and separated sample components(analytes) exit the GC and combine with a make-up gas flow 134 beforeentering the flow cell 104 of the VUV absorption detector via the inletport 135. The column, make-up gas stream, and flow cell are maintainedat an elevated temperature to prevent condensation of the elutedspecies. The gases entering the flow cell travel the length of the celland exit unconsumed via the outlet port 140 at the other end of thecell. Both the inlet and outlet ports are equipped with standard GCfittings so as to minimize “dead-volume”.

The collimated light beam entering the flow cell passes through the gasstream traveling along the flow channel. Eluted components absorb lightfrom the light beam resulting in reduced transmission and a detectablesignal. The detected signal (essentially the transmittance through theflow cell) is recorded as a function of time and is dependant on theidentity and density of analytes present in the light beam.Fortuitously, typical carrier gases employed in GC work (i.e. hydrogen,helium, and nitrogen) do not significantly absorb light in the VUV;remaining essentially invisible to the detection system.

The light beam transmitted through the flow cell passes through anotherVUV transparent window 142 at the end of the cell and is focused by asecond VUV optic 150 onto the entrance aperture 152 of the spectrometer.Light passing through the aperture is collected, diffracted, and focusedby a grating 154 onto a detector 156 where it is recorded by a computer158.

In one embodiment, an aberration corrected flat field diffractiongrating is employed to simultaneously focus and diffract the collectedlight; thereby reducing the number of optical elements required, andimproving optical efficiency. Similarly, the use a wide dynamic range,highly sensitive, back-thinned CCD image sensor may prove particularlyadvantageous. Typically, the detector electronics 160 are housed outsidethe detector module chamber and are connected via an electrical feedthrough in an effort to minimize contamination sources inside theinstrument.

Just as with the source module, the environment within the detectormodule is also controlled via gas connections 162 so as to minimize theconcentration of VUV absorbing species. Similarly, the exit end of theflow cell is connected to the detector module by means of a thermalisolation coupling 164 and appropriate leak tight seals. While notexplicitly shown, the entire system (i.e. source, shutter, gasconnections, detector, etc) may be controlled by a software programrunning on a computer and/or embedded controller.

An expanded view of the flow cell 104 is provided in FIG. 2. Thermalisolation couplings 122 and 164 (in this case ceramic) on either end ofthe cell are readily apparent; as are the VUV windows 118 and 142 andtheir corresponding seals 204. Also evident in the figure are GCfittings 202 as known in the art to connect the column gases 133 andmake-up gases 134 to the inlet port of the flow cell. The end of thecolumn from the GC can be seen extending down to the outer diameter ofthe flow channel.

As the flow cell is expected to expand measurably upon heating, thesystem must be designed in such a manner as to accommodate saidexpansion without adversely affecting the optical alignment of thesystem or introducing unwanted mechanical deformations. While notexplicitly represented in FIG. 1, one means of accomplishing this is torigidly mount the detector module on a base plate, while at the sametime mounting the source module on an optical rail mounted to the samebase plate (alternatively the source module may be rigidly mounted andthe detector module placed on an optical rail or even both modules maybe on an optical rail). In this manner relative motion between themodules can be restricted to a single axis running collinear with theoptical path of the flow cell. As such, increases in the length of theflow cell induced through heating will cause the source module to travelaccordingly along the length of the rail without affecting opticalalignment or introducing mechanical deformations. Conversely, whenheating is discontinued and the flow cell cools it will shorten causingthe source module to again move accordingly along the optical rail.

The geometry of the flow cell plays an integral role in the signaldetected by the system. Explicitly, the intensity of light when a singletype of analyte is in the cell is given by:

$\begin{matrix}{{I(\lambda)} = {{I_{o}(\lambda)}e^{{- {\sigma{(\lambda)}}}\frac{L\; N}{V}}}} & {{Eqn}.\mspace{14mu} 1}\end{matrix}$

-   -   where I_(o)(λ) is the intensity of the light when no analyte is        in the cell, σ is the absorption cross section (per molecule) of        the analyte, L is the cell length, N is the number of analyte        molecules in the cell, and V is the cell volume.

To enable the highest possible absorption response from a given analyteto be recorded it is desirable to ensure that all of the analytemolecules are contained within the flow cell long enough for theircollective absorbance to be measured. While maintaining this condition,it is further desirable to reduce the cross-sectional area of the cellas much as possible so as to increase the density of analyte molecules.Manufacturing, optical and analytical considerations pose practicallimitations on the extent to which this is possible.

In situations where larger flow cell volumes are employed, make-up gasmay be combined with column effluent via the inlet fitting to aid inpeak separation; hence maintaining the temporal resolution of thecolumn. The make-up gas flow is configured such that it is computercontrolled and can be adjusted in real-time to enhance systemperformance. Any gas which does not significantly absorb VUV light (i.e.nitrogen, helium, hydrogen, etc.) may be used as make-up gas. It followsthat the addition of make-up gas does not adversely affect detectorsensitivity, provided the flow is not so high that analyte molecules arerushed out of the cell before they can be measured. This is not the casewith other GC detectors (i.e. FIDs, TCDs, etc) wherein changes inmake-up gas flow directly impact detector sensitivity. At times it maybe desirable to introduce low concentrations of certain species into themake-up gas to promote cleaning of optical surfaces and/or prevent thebuild up of solarized compounds on such.

While not represented in FIG. 2, both the flow cell and the portion ofthe GC column running between the GC and the detector are equipped witha means of heating. Typically these are well insulated and maintainedabout 20° C. higher than the temperature of the GC oven. The temperatureof both the exposed column and flow cell are computer controlled. Incases where reactive species are to be studied, an inert coating may beapplied to the inside of the flow channel. Alternatively, the flow cellitself may be largely constructed of an inert material.

In operation the gas stream exiting the column travels down the flowchannel 210 and exits the flow cell via the outlet port 140. The outletcan be vented to atmosphere or connected to vacuum. The exiting gasstream can also be introduced to another detector. While simplyrepresented in the figure, the geometry of the flow cell and associatedGC fittings may include certain features specifically designed topromote laminar flow and reduce, or altogether remove, “dead volume”.

FIG. 3 presents another embodiment of the disclosed systems. In thisconfiguration the flow cell 104 is positioned in a dedicated chamber 302between the source module 102 and detector module 106. It is noted thatin this embodiment the flow cell is not rigidly attached to the sourceand detector modules via thermal isolation couplings, but is insteadequipped with a second set of VUV windows 306. While not explicitlyshown in the figure the flow cell is instead supported via thermalisolation standoffs attached to a base plate and optical rail assemblyas described earlier. These standoffs conduct much less energy than thecouplings depicted in FIG. 1 since they are not involved in sealing.This arrangement facilitates heating of the flow cell to highertemperatures without risk of heating the remainder of the system.

The environment within the flow cell chamber is also maintained so as topermit the transmission of VUV photons across the gaps between thesource and detector module windows and that of the flow cell. Theability to control the environment within this volume (for examplethrough ports 308), independent of the source and detector modules mayprove useful in light of the increased concentration of contaminantswhich may result due to the elevated temperatures at play.

Yet another embodiment of the system is presented in FIG. 4. Unlike thecollimated beam systems of FIGS. 1 and 3, the system of FIG. 4 employsfocusing optics and a flow cell with a smaller cross-sectional area. Thesmaller cross-sectional area increases the density of analyte molecules(thus increasing the absorption signal) but as a consequence alsorestricts the photon flux achievable using a collimated beam. As aresult, the flow cell chamber 404 houses VUV optics in addition to theflow cell. The first optic 406 receives the collimated beam from thesource module 102 and focuses it onto the entrance window 408 of theflow cell. Since at least some of the rays entering the flow cell areexpected to reflect from its inner walls, care must be taken to ensurethe walls reflect said rays efficiently. The required reflectivitydepends to a large extent on the f-number of the focusing optic and thecell geometry. Depending on the choice of these variables, coated(internally and/or externally) and/or uncoated cells may be employed.

While the body of the flow cell may be constructed of a variety ofmaterials, chemically inert glasses like fused silica may be preferred.The entrance and exit windows on the cell may be attached usingdedicated seals (as described earlier), fusing, or with appropriate lowout-gassing cement and/or epoxy. Similarly, the gas inlet and outletports may be fully or partially formed during construction of the cellor may be added after the fact.

Light passing through the cell exits through the second VUV window 410and is collected by a collimating optic 412 which directs it to thefocusing optic 150 in the detector module as previously described. Whileother combinations of optics could certainly be used to direct lightthrough the system, arrangements which send focused beams throughbirefringent windows are avoided whenever possible so as to circumventchromatic aberrations.

As with the embodiments of FIGS. 1 and 3, the embodiment of FIG. 4 isalso configured so as to facilitate flow cell heating, and isconstructed in such a means as to accommodate the resultant thermalexpansion.

Before the embodiments of FIG. 1, 3 or 4 can be used to obtain achromatogram, it is desirable to first record both “dark” and“reference” spectra. The “dark” spectrum corresponds to the signalobtained in the absence of light. It is simply collected by closing theshutter in the source module and recording the background level seen bythe detector. Once measured, this “dark” spectrum is subtracted from allother spectra (both reference and sample spectra) prior to determiningtransmittance and/or absorbance.

The “reference” spectrum is obtained in a similar fashion by opening theshutter and recording the intensity through the cell prior to injectionof a sample (i.e. in the presence of only carrier gas flow) to obtain a“light” spectrum. Subtraction of the “dark” spectrum from the “light”spectrum yields the “reference” spectrum which represents the intensityof light when no analyte is in the cell, I_(o)(λ), presented earlier inEqn. 1.

Since both the “dark” and “reference” spectra are obtained once prior tosample injection they are typically time averaged to some extent inorder to improve the signal to noise ratio of the data. If the detectoris temperature controlled it is likely the “dark” spectrum will notchange appreciably; if it is not it may be necessary to update the“dark” spectrum when changes in the ambient temperature are experienced.Meanwhile, the “reference” spectrum may be influenced by many factorsincluding, but not limited to; changes in ambient temperature andpressure, changes in the environments of the system modules, changes inthe optical system and changes in source output. These variations may beaccounted for by collection of a new “reference” spectrum, or throughempirical corrections.

With both “dark” and “reference” spectra in hand, the transmittancethrough an injected sample can be readily calculated as:

$\begin{matrix}{{T(\lambda)} = \frac{I(\lambda)}{I_{o}(\lambda)}} & {{Eqn}.\mspace{14mu} 2}\end{matrix}$

-   -   Similarly, the absorbance can be expressed as:

$\begin{matrix}{{A(\lambda)} = {{\log\left( \frac{1}{T} \right)} = {\log\left( \frac{I_{o}(\lambda)}{I(\lambda)} \right)}}} & {{Eqn}.\mspace{14mu} 3}\end{matrix}$

-   -   For a single analyte in a sample cell of length L and volume V,        the transmittance in Eqn. 2 can be further expressed as

$\begin{matrix}{{T(\lambda)} = e^{{- {\sigma{(\lambda)}}}\frac{N}{V}L}} & {{Eqn}.\mspace{14mu} 4}\end{matrix}$

-   -   where N is the number of analyte molecules present in the cell,        and σ(λ) is the wavelength-dependent absorption cross section        per molecule, usually just referred to as absorption cross        section, and expressed in units of area. In addition to        depending on wavelength, the absorption cross section is        different for different analytes. The wavelength-dependent        absorption cross section is the “fingerprint” that enables        selectivity when using optical spectroscopy.

Alternately, the absorption by an analyte in the cell can becharacterized by the absorbance of Eqn. 3 expressed as:

$\begin{matrix}{{A(\lambda)} = {{\log_{10}\left( \frac{1}{T} \right)} = {\frac{1}{\ln(10)}{\sigma(\lambda)}\frac{N}{V}{L.}}}} & {{Eqn}.\mspace{14mu} 5}\end{matrix}$

-   -   In the case where a single component of known cross section is        in the sample cell, Eqn. 5 can be directly inverted to obtain        the number of analyte molecules in the cell:

$\begin{matrix}{N = {\frac{V\;{\ln(10)}}{{\sigma(\lambda)}L}{{A(\lambda)}.}}} & {{Eqn}.\mspace{14mu} 6}\end{matrix}$

In principle, only the absorbance and cross section at one wavelengthvalue is needed in order to determine N, although in practice data frommultiple wavelengths can be used via a regression procedure, with theadvantage of reduced uncertainty in the determination of N. Alternately,the inversion in Eqn. 6 can be performed for each measured wavelengthvalue, and the N obtained verified for consistency. Different N obtainedusing data at different wavelengths implies an error in the measureddata, or that the wavelength-dependence of the assumed cross section isin error.

Typically, the molar mass, M, of the analyte is known, and this can beused to calculate the mass of analyte in the sample cell via

$\begin{matrix}{{m = {\frac{M}{N_{A}}N}},} & {{Eqn}.\mspace{14mu} 7}\end{matrix}$

-   -   where N_(A) is Avogadro's constant. Therefore, with knowledge of        the analyte cross section and the cell geometry, a chromatogram        can be converted to either number of molecules or mass in the        cell as a function of time. The cell geometry can also be        invoked in order to express the number density or mass density        of analyte in the cell. A concentration can be computed by        knowing the injected solvent volume (e.g., micrograms per        milliliter of solvent).

For a case involving multiple analyte components in the sample cell at agiven time, the absorbance is given by

$\begin{matrix}{{{A(\lambda)} = {\frac{L}{V\;{\ln(10)}}{\sum\limits_{i = 1}^{n}{{\sigma_{i}(\lambda)}N_{i}}}}},} & {{Eqn}.\mspace{14mu} 8}\end{matrix}$

-   -   where n is the total number of analyte components in the cell,        σ_(i)(λ) is the absorption cross section of component analyte i,        and N_(i) is the number of molecules of component i. A situation        like this may arise when a solution consisting of many        components is directly injected into the sample cell or in cases        where multiple components arrive at the VUV spectrometer        simultaneously during a GC analysis (i.e., where the components        coelute).

Solving Eqn. 8 for the unknown N_(i) requires absorbance measurements atleast n different wavelength values. In this case, Eqn. 8 is a system ofn linear equations, which can be solved using techniques known in theart. In practice, Eqn. 8 is over-determined as there are many more datapoints than unknown quantities N_(i). Such an equation can be reduced toa number of independent equations equaling the number of unknowns.Alternately, a regression fitting technique can be used. A regressiontechnique is also advantageous in that it allows for uncertainty in themeasured data, as well as in the assumed cross sections. The result ofthe regression of Eqn. 8 is a set of best fit values for the N_(i) aswell as a confidence metric, often called a “Goodness Of Fit” (GOF). Onesuch regression technique is the Levenberg-Marquardt method described inPress, et al. (W. H. Press, S. A. Teukolsky, W. T. Vetterling, and B. P.Flannery. Numerical Recipes in C: The Art of Scientific Computing,Second Edition. Cambridge University Press, 1992).

Thus, given a set of analyte components whose wavelength-dependentabsorption cross sections are known, a measurement of thewavelength-dependent absorbance spectrum can be used to determine theset of N_(i) most consistent with the measured spectrum—i.e., theunknown amounts of each analyte component can be determined.

In a case where the cross section values are not known, Eqn. 5 cannot besolved for all of the unknowns, since there is now an unknown crosssection for each measured data point, plus one additional unknown, N.More generally, it is desirable to store a database of cross sectionvalues for various “known” substances that can be used as inputs toEqns. 5, 6, and 8 when measuring amounts of various analytes.Furthermore, as the wavelength-dependent cross section is essentiallythe identity of an analyte, it is advantageous to be able to search aset of absorbance data (e.g. from a VUV spectroscopic chromatogram) forthe presence of a particular analyte. Accordingly, a method or methodsfor determining the cross section spectrum when it is not already knownis desirable.

In a first procedure, the absorbance is measured for a known amount ofthe analyte. A convenient way to accomplish this procedure is to combinea known quantity of sample with a solvent, inject the mixture into a GCinjection port, and measure the eluate with a VUV detector. The GCseparation ensures that the analyte component is measured by itself.Then the cross section can be determined at every wavelength for whichthere is absorbance data:

$\begin{matrix}{{\sigma(\lambda)} = {\frac{V\;{\ln(10)}}{NL}{{A(\lambda)}.}}} & {{Eqn}.\mspace{14mu} 9}\end{matrix}$

This procedure need only be performed once for a given analyte. Anunknown amount of the analyte can later be determined using the methodsdiscussed above by making use of the now known cross section, regardlessof whether the analyte is measured alone or together with other analytecomponents whose cross sections are also known.

It is noted that if a GC separation is utilized in the above manner, thecarrier gas and/or VUV detector makeup gas may need to be adjusted inorder to ensure that an absorbance spectrum is obtained when all of theanalyte is present in the sample cell, so that the value assumed for Nin Eqn. 9 corresponds to the amount of analyte in the injected sample.This can usually be accomplished by simply slowing the detector makeupgas flow, but optimizing the GC settings may also be required.

A drawback of the first method is that uncertainties in the amount ofanalyte reaching the sample cell can affect the accuracy of thedetermined cross sections. For example, there can be variations in thevolume of sample injected, sample can leak out of the system, errors canexist in the split flow calibration, etc. In such cases where sample isindiscriminately lost, a second procedure for determining the unknownanalyte cross section is now described.

Let N_(a) be the number of analyte molecules injected, and N_(S) thenumber of solvent molecules. Since the densities and molecular massesare typically known, N_(a) and N_(S) can be determined from the knownvolume or mass ratios of the sample and the total injected volume. Theabsorbance of the analyte is given by

$\begin{matrix}{{A_{a}(\lambda)} = {\frac{1}{\ln(10)}{\sigma_{a}(\lambda)}\frac{N_{a}}{V}L}} & {{Eqn}.\mspace{14mu} 10}\end{matrix}$

-   -   and that of the solvent by

$\begin{matrix}{{A_{S}(\lambda)} = {\frac{1}{\ln(10)}{\sigma_{S}(\lambda)}\frac{N_{S}}{V}{L.}}} & {{Eqn}.\mspace{14mu} 11}\end{matrix}$

-   -   The solvent cross section, σ_(S), is taken to be known. Taking        the ratio of the absorbances yields

$\begin{matrix}{{A_{r}(\lambda)} = {\frac{A_{a}(\lambda)}{A_{S}(\lambda)} = \frac{{\sigma_{a}(\lambda)}N_{a}}{{\sigma_{S}(\lambda)}N_{S}}}} & {{Eqn}.\mspace{14mu} 12}\end{matrix}$

The analyte and solvent components are separated by the chromatographyprocess, so the absorbances for analyte and solvent can be measuredseparately. Thus, A_(r) can be measured by applying Eqn. 12 to themeasured A_(a) and A_(S) for each wavelength. The result for A_(r) is aspectrum of absorbance ratios having the same number of data points aseach of A_(a) and A_(S). Since N_(a)/N_(S) and σ_(S) are known, Eqn. 12can be solved for σ_(a):

$\begin{matrix}{{\sigma_{a}(\lambda)} = {{\sigma_{S}(\lambda)}\frac{N_{S}}{N_{a}}{{A_{r}(\lambda)}.}}} & {{Eqn}.\mspace{14mu} 13}\end{matrix}$

Therefore, as long as the solvent cross section and relative proportionof analyte to solvent molecules are known, the cross section for theanalyte is determined at every wavelength value for which there ismeasured absorbance data. It is noted that the solvent has to absorb(have a nonzero cross section) for wavelength regions of interest inorder to use this method. Most solvents absorb throughout the VUVregion. If the analyte doesn't absorb in a particular wavelength region,the analyte cross section there is trivially zero, although this resultwould also fall out in the above analysis.

An advantage of this method is that only N_(a)/N_(S) needs to be known:variations in N_(a) and N_(S) are allowed as long as N_(a)/N_(S) isunaffected. Therefore systematic errors during a gas chromatographymeasurement and separation process that result in variations in thevolume of solution reaching the detector do not affect the ability todetermine the unknown cross section of the analyte. Examples of suchvariations may include variations/uncertainties in injection volume,variations/uncertainties in split flow, variations in transferefficiency from the injector to column, leaks, etc. The procedure todetermine the analyte cross section need be performed only once, afterwhich the analyte is “known” to the system (i.e., stored in a crosssection library), and the wavelength-dependent cross section isavailable for subsequent measurement of solutions containing an unknownamount of the analyte, whether occurring alone or in a mixture of othersubstances.

In one embodiment, the ratio N_(a)/N_(S) is taken to be the same as theinjected ratio. As before, the measured absorbances used in Eqns. 12 and13 should correspond to the case where all of the analyte and/or solventis present in the sample cell. The makeup gas flow can be slowed so atleast one absorption scan will be obtained when all of the analyte orsolvent molecules are in the detector sample cell. Since a typicalchromatogram will consist of multiple absorption scans for each of theanalyte and solvent, a straight-forward criterion is to use themeasurements from the analyte and solvent peaks that correspond tomaximum absorption for each of those peaks.

An example is presented to illustrate this second method. A mixtureconsisting of 3 parts (by volume) ethanol and 5 parts methylene chloridewas made. For this illustration, methylene chloride is taken to be theknown solvent, and ethanol to be the analyte with unknown cross section.The densities, molar masses, and other general properties are assumed tobe known for both ethanol and methylene chloride. A 0.1 μL volume of thesolution was injected into a GC/VUV spectroscopy system. From the knowndensities and molar masses, the total amount of ethanol (e.g., number ofmolecules or total mass) and methylene chloride injected can bedetermined. From the GC split ratio, the amount of the injected ethanoland methylene chloride that should in principle reach the detector canbe calculated. In practice, the amount observed at the detector usingabsorption measurements can be somewhat different (usually the amount isless), and this is due to variations in injection volume, accuracy ofsplit flow and split ratio measurements, losses due to leaks, etc. Theratio of the amount of ethanol to the amount of methylene chloride ismore reliable:

$\begin{matrix}{\frac{N_{EtOH}}{N_{{CH}_{2}{Cl}_{2}}} = {{\frac{3}{5}\left( \frac{\rho_{EtOH}}{\rho_{{CH}_{2}{Cl}_{2}}} \right)\left( \frac{M_{{CH}_{2}{Cl}_{2}}}{M_{EtOH}} \right)} \approx 0.6562}} & {{Eqn}.\mspace{14mu} 14}\end{matrix}$

-   -   where ρ stands for density and M for molar mass. The factor of ⅗        in Eqn. 14 comes from the volume mixing ratio of 3:5, and would        in general be different for different mixing ratios. The ratio        in Eqn. 14 does not depend on injected volume, nor any loss        mechanism that changes the total volume of sample        indiscriminately.

FIG. 5 shows a chromatogram from a 0.1 μL split injection of the 3:5solution using a split ratio of 100:1. Each data point corresponds to acomplete absorbance spectrum over the VUV wavelength region. Theresponse plotted in FIG. 5 is the absorbance averaged over the 125-220nm wavelength region. The peaks are labeled in FIG. 5—the two componentsare well-separated with the EtOH component eluting around 29 seconds andthe methylene chloride around 35 seconds.

The EtOH signal appears to tail slightly into the methylene chlorideregion, implying that the methylene chloride signal is at least a little“contaminated” by the EtOH left in the cell. In most actual cases ofinterest, the separation would be much greater (in this case by simplydecreasing the carrier gas velocity), and each of the two componentswould occupy the cell by themselves. For this illustrative example, itis assumed that the CH₂Cl₂ spectrum consists of a singlecomponent—although the ethanol cross section that results willpresumably suffer a small amount of error due to this assumption.

The “chopped-off” appearance of the peaks is due to slowing the makeupgas in order to ensure that at least one spectrum is obtained where theentire amount of each component is present in the sample cell. Thisspectrum is taken to be the one corresponding to maximum absorption foreach of the ethanol and methylene chloride peaks.

FIG. 6 shows the 125-200 nm absorbance spectra from the maximumabsorbance scans for EtOH and CH₂Cl₂, corresponding to 29.6 seconds and35.3 seconds, respectively. FIG. 7 (top) shows the absorbance ratio(A_(r) of Eqn. 12), generated by dividing the ethanol absorbance by themethylene chloride absorbance on a wavelength-by-wavelength basis. FIG.7 (bottom) shows the ethanol cross section generated by multiplying theabsorbance ratio by the methylene chloride cross section, again on awavelength-by-wavelength basis. It is noted that nothing was assumedabout the ethanol cross section, which has now been determinedthroughout the measured wavelength region. The cross section isavailable for use in subsequent measurements of samples containingunknown amounts of ethanol.

In order to equate N_(a)/N_(S) in Eqn. 13 with the value of the ratio oninjection, it is preferable to measure absorbance for each of thesolvent and analyte at times when all of the solvent/analyte moleculesare in the sample cell. However, it is conceivable that a ratio ofN_(a)/N_(S) corresponding to solvent and analyte absorbances when notall of the solvent or analyte molecules are present in the cell could bedetermined and used in the above method. The method will work the sameway as long as the absorbance spectra used to form the absorbance ratioin Eqns. 12 and 13 really are the ones that correspond to the determinedvalue of N_(a)/N_(S).

It is noted that although the terminology of “solvent” and “analyte” areused above, the method can be applied to any two components of a mixturewherein the cross section of one is known, and the relative amount ofthe two components is known. The sample could consist of a solvent withmultiple easily separated analytes, such as is often provided by vendorsof standard samples. The known component could be any of the analyteswhose cross section is known. This component then serves as the“solvent” in the above method. The method can then be applied to any ofthe unknown components, which becomes the “analyte”. Normally therelative amounts of components are known in these standard samples, soknowledge of the cross section for one component allows fordetermination of the cross sections for the remaining components,provided the condition of measuring each isolated component when all ofthe component molecules are in the sample cell is met. In fact, whencombined with the next method described below, knowledge of a crosssection value at a single wavelength for one of the components may beenough to determine the cross section spectra for all of the componentsin the standard sample.

A third method for determining an unknown analyte cross section is nowdescribed. It is convenient to re-write Eqn. 5 for this purpose. Evenwhen the measured wavelength values are closely spaced, the measuredabsorption spectrum is typically discrete, having been determined usinga photodiode or CCD array. For the following, the explicit wavelengthdependence in Eqn. 5 is replaced with an index:

$\begin{matrix}{A_{j} = {\frac{1}{\ln(10)}\sigma_{j}\frac{N}{V}{L.}}} & {{Eqn}.\mspace{14mu} 15}\end{matrix}$

The values of j are integers, for example, j=1 may correspond to thelowest wavelength value measured, j=2 to the next lowest, and so on. Thevalue of σ_(j) is the value of the absorption cross section at thewavelength corresponding to the index j. As an example, the absorbanceaveraged over a particular wavelength region becomes

$\begin{matrix}{A_{{int},{norm}} = {\frac{1}{n}{\sum\limits_{j = j_{m\; i\; n}}^{j_{{ma}\; x}}A_{j}}}} & {{Eqn}.\mspace{14mu} 16}\end{matrix}$

-   -   where j_(min) is the index corresponding to the lowest        wavelength in the integration region and j_(max) to the index of        the highest wavelength value in the integration region. In Eqn.        16 n=j_(max)−j_(min)+1.

Given an absorbance spectrum for a single analyte component as in Eqn.15, the values A_(j) can be normalized by dividing by A_(j) for aparticular value of j, say j_(norm). The result is

$\begin{matrix}{A_{j,{rel}} = {\frac{A_{j}}{A_{j_{norm}}} = \frac{\sigma_{j}}{\sigma_{j_{norm}}}}} & {{Eqn}.\mspace{14mu} 17}\end{matrix}$

The ratio in Eqn. 17 depends only on the ratio of cross sectionvalues—all wavelength-independent quantities cancel out. This means thatthe relative absorbance for a single analyte is unique to thatparticular analyte, and does not depend on the cell geometry or theamount of analyte present. Furthermore, Eqn. 17 shows that the relativecross section is determined by a single absorption spectrum measurement.If the absolute value of the cross section is known for even a singlewavelength within the measured wavelength region, the absolute crosssection is determined for the entire region:σ_(j)=σ_(j) _(norm) A_(j,rel),  Eqn. 18

-   -   assuming the known cross section is σ_(j) _(norm) itself. If the        cross section is known at a different wavelength value,        j_(known), then the σ_(j) can be determined from

$\begin{matrix}{\sigma_{j} = {\sigma_{j_{known}}A_{j,{rel}}{\frac{A_{j_{norm}}}{A_{j_{known}}}.}}} & {{Eqn}.\mspace{14mu} 19}\end{matrix}$

-   -   The ratio could also have been formed in the first place using        j_(norm)=j_(known), i.e., by using the absorption at the        wavelength of the known cross section as the normalizing value        in Eqn. 17.

The known cross section value can be an accepted literature value forthe cross section. There are many instances where cross sections areknown for one or a small number of wavelength values in the VUV region,but unknown otherwise. In addition, absorption measurements could beextended into the UV or visible wavelength regions to encompass regionswhere there are known cross sections. This third procedure can be usedin either case to determine the cross section values throughout the restof the VUV region.

In cases where there is no cross section information available, theprocedure detailed in Eqns. 10-13 can be employed to determine the crosssection at a single wavelength value, and then Eqns. 18 or 19 can beused to determine the cross section throughout the rest of themeasurement region. There are several advantages to this approach. In acase where a solvent/analyte solution can be made and measured, thesolvent absorption may be strong enough to saturate some wavelengthregions (i.e., transmittance through the solvent at those regions dropsto zero). Eqns. 10-13 can be used to determine a cross section at thosewavelengths where the solvent transmittance is not zero, and a relativeabsorption as in Eqns. 17-19 used to determine the rest of the crosssection values. The relative absorption measurement used to determinethe rest of the cross section spectrum does not have to come from thesame set of measurements, so the GC run parameters can be optimizedspecifically for analyte signal in order to improve signal to noisecharacteristics and reduce the effects of measurement uncertainty on thedetermined cross section spectrum. The GC run parameters for this secondrun could also be optimized for convenience, since Eqns. 17-19 can beused for any absorbance measurement where the analyte occurs in thesample cell by itself. Otherwise, the absorbance does not have tocoincide with a condition where every analyte molecule is simultaneouslyin the sample cell. One way to maximize the amount of analyte introducedto the system is to perform a splitless injection (no split flow). Insuch cases, it may be hard to ensure that all of the analyte moleculesare in the sample cell at any particular point in time, but a splitlessor low split ratio injection may still provide the largest possibleanalyte response.

The procedure employing Eqns. 10-13 can be used to find an analyte crosssection at a wavelength where the absorbance properties of analyte andsolvent are optimal, reducing the effects of measurement uncertainty onthe determination of the single wavelength cross section. This optimalcondition may be easier to achieve at one particular wavelength valuethan for the entire measured region simultaneously. A second measurementoptimized specifically for the unknown analyte can be used with Eqns.17-19 to determine the rest of the cross section spectrum, thusimproving the accuracy and minimizing uncertainty in the cross sectionfor the entire wavelength region.

A fourth method again isolates and measures absorbance or transmittanceof an analyte. Whatever literature or otherwise known values that existfor cross sections are used in a fit procedure using data just fromthose wavelengths where the cross section is known. For example, crosssection data may be available at wavelengths above 180 nm but notfurther into the VUV. The result of the fit is a value for the amount ofthe analyte N. This value for N is then used in Eqn. 9 to obtain thecross section values at the remaining unknown wavelengths or wavelengthregions.

While the obtained absorbance/cross section data is discrete, thewavelength spacing is typically small (<1 nm). If absorbance or crosssection data between successive wavelength values is needed, thesevalues can be determined by interpolation.

An exemplary aspect of the VUV spectroscopy system is that theavailability of absorption cross sections for analytes enablesquantitative analysis from absorption/transmittance measurements withoutthe need for explicit calibration using known samples.Absorbance/Transmittance measurements can be used to determine theamount of analyte in the sample cell at a given time, independent offactors affecting a GC separation process. If there are losses of sampleduring the chromatography process, these errors will show up asdifferences in the amounts of each analyte detected, even in cases wherethe relative amounts are unaffected. Because of this, GC systemefficiency can be characterized through use of a known standard, andvariations in the detected amount of the standard are attributable tovariations in the efficiency of the injection, transfer, and/orseparation process. The standard can also be used to compareefficiencies of different GC systems. The ability to do this is notcommon. Typical GC detectors require frequent calibration using a seriesof known samples to do any kind of quantitative analysis, and aside frombeing time-consuming; such calibration processes reveal no informationabout the efficiency of the GC process. In cases where errors that arenot indiscriminant are suspected (i.e., errors that depend on thespecific analyte), a standard can be prepared with known amounts ofmultiple components, and variations in the relative amounts of thecomponents monitored using the VUV detector.

A second advantage is that since the absorption cross section spectrumis unique to a particular analyte, an inherent selectivity exists whenusing a VUV spectroscopy detector. Using absorption cross sectionsstored in a database library, a relative cross section as defined byEqn. 17 can be compared to relative absorption spectra from atime-dependent chromatogram. If a particular relative absorptionspectrum was due to the analyte in question, an exact match (to withinmeasurement uncertainty) will result, regardless of the amount ofanalyte measured. Thus the presence of the analyte spectrum is detectedin the chromatogram. It is not unreasonable that an entire librarydatabase of cross sections could be searched this way, and a closestmatch assigned to each chromatogram peak. Note that the normalizationwavelength should be the same for both the relative cross section andrelative absorbance being compared, although it is possible to accountfor the constant offset factor that may result if the normalizationwavelengths are different.

If the possibility of coelution exists, a regression fit procedure canbe performed on the absorbance data using Eqn. 8 (or the analogousequation for transmittance). Inputs include cross sections for candidateanalytes. Those analytes that return fit amounts, N_(i), substantiallydifferent from zero are likely present in the sample cell at the timecorresponding to that particular absorbance spectrum. This procedureresults in a determination of the amounts of the detected analytes aswell.

During the fitting procedure, the number of candidate analyte componentswould probably be reduced to the extent possible to avoid potentially(and unnecessarily) fitting thousands of components simultaneously. Thesearch interface can provide a means of identifying likely componentsaccording to the possibility of their presence in the solution beinginjected. Known retention times can also be used, and candidate analytesincluded in the search/fit only in regions of the chromatogram wherethey are likely to occur. A means to provide retention times forspecific analytes and a time window around the retention times can beincluded. Fits involving regions of the chromatogram that fall insidethe window would include a term for the analyte in question.

In one embodiment, the output of the VUV detector is an absorbance ortransmittance spectrum. As has already been illustrated, time dependentprocesses can be monitored by recording absorbance or transmittance atregular intervals. The dataset provided in this case isthree-dimensional in nature, consisting of absorbance (or transmittance)as a function of both wavelength and time. When coupled with a gaschromatograph, the time-dependence of the absorbance or transmittancespectrum is recorded as various eluates exit the column and pass throughthe detector sample cell. While each recorded data point is a spectrumover the entire measured wavelength region, it is convenient to generatea total detector response that correlates with the eluates exiting theGC column. This total response provides a convenient two-dimensionalview of the data that can be presented as a chromatogram.

It was pointed out that a typical absorbance/transmittance spectrum isdiscrete; the transmitted light having been collected using an arraydetector. A total response can consist of a sum of theabsorbances/transmittances at each wavelength value. For the case ofabsorbance:

$\begin{matrix}{A_{int} = {\sum\limits_{j = 1}^{n}A_{j}}} & {{Eqn}.\mspace{14mu} 20}\end{matrix}$

-   -   where n is the total number of wavelength values. The response        in Eqn. 20 can be normalized by dividing by the total number of        data points:

$\begin{matrix}{{A_{{int},{norm}} = {\frac{1}{n}{\sum\limits_{j = 1}^{n}A_{j}}}},} & {{Eqn}.\mspace{14mu} 21}\end{matrix}$

-   -   which is the same as the average absorbance over the measured        wavelength region. In addition to the total response in Eqns. 20        or 21, the response over a particular wavelength region can be        computed:

$\begin{matrix}{{A_{int} = {\sum\limits_{j = j_{m\; i\; n}}^{j_{{ma}\; x}}A_{j}}},} & {{Eqn}.\mspace{14mu} 22}\end{matrix}$

-   -   where j_(min) is the index corresponding to the lowest        wavelength in the integration region and j_(max) to the index of        the highest wavelength value in the integration region. If an        average absorbance over the region in question is desired, then

$\begin{matrix}{A_{{int},{norm}} = {\frac{1}{n}{\sum\limits_{j = j_{m\; i\; n}}^{j_{{ma}\; x}}A_{j}}}} & {{Eqn}.\mspace{14mu} 23}\end{matrix}$

-   -   where n=j_(max)−j_(min)+1. Eqn. 23 was also presented earlier as        Eqn. 16.

Eqn. 20 can be modified to express an area under the absorbance versuswavelength curve:

$\begin{matrix}{A_{int} = {\sum\limits_{j = 1}^{n}{A_{j}{\Delta\lambda}}}} & {{Eqn}.\mspace{14mu} 24}\end{matrix}$

-   -   where Δλ is the spacing between successive wavelength values,        which are assumed to be approximately equally spaced. A_(int) in        Eqn. 24 has units of absorbance units times length. Eqn. 24 is        closer to the discrete version of an integral—however,        especially in the case of constant wavelength spacing, Eqn. 20        contains the same information, so the nomenclature A_(int) was        retained there. Note that when the average absorbance is        calculated, Eqn. 24 still becomes Eqn. 21 (or Eqn. 23 if a        specific wavelength region is considered). If the wavelength        values are not equally spaced, Δλ can also be indexed, and the        spacing appropriate to each adjacent set of data points used in        Eqn. 24.

If the absorbance data in Eqns. 20-24 are continuous, the sums can beconverted to integrals. Any of Eqns. 20-24, discrete or continuousversions, or any number of further variations can be reported as adetector response on a chromatogram. The actual wavelength-dependentabsorbances can still be stored, and invoked for full data analysiswhenever needed. The detector responses can also be applied totransmittance data.

The following discussion uses Eqn. 23 applied to absorbance spectra forthe detector response, although these choices are strictly forillustrative purposes. Each measurement in a chromatogram consists of anabsorbance spectrum over the entire measured wavelength region. Inaddition, different analytes generally have absorption cross sectionswith different wavelength characteristics. It may be beneficial to applyEqn. 23 to a wavelength region tailored to the wavelengthcharacteristics of a particular analyte or class of analytes. Such aspectral filter will enhance the chromatogram response to that class ofanalytes, and provides a level of selectivity in and of itself. It ispossible to construct multiple chromatograms using multiple filters froma single GC/VUV detector run, or to return to a chromatogram datasetlater and re-analyze it with different filters.

FIG. 8 shows an example of a GC/VUV measurement of unleaded gasoline. A125 nm-220 nm filter is applied, covering most of the measuredwavelength range. FIG. 9 compares 125 nm-240 nm absorbance spectra foran aliphatic hydrocarbon, most likely iso-octane (top), and toluene(bottom). The response from aliphatic hydrocarbons is generallyconcentrated in the far-VUV region. Toluene is an aromatic compound—theresponse for these compounds spans the VUV region, but have maximalresponse in the 180-190 nm region. FIG. 10 (top) shows the samechromatogram as in FIG. 8, but with a 150 nm-200 nm filter applied.Several aromatic compounds are labeled. It is seen from this figure thatthe response from aliphatic hydrocarbons is suppressed. FIG. 10 (bottom)shows the chromatogram with a 125 nm-160 nm filter applied, whichsuppresses the aromatic compound response in favor of the aliphatichydrocarbons. A more dramatic example is given in FIG. 11 (top), where a200 nm-220 nm filter has been applied. This filter suppresses bothaliphatic hydrocarbons and aromatics in favor of polycyclic aromatichydrocarbons (PAHs). Naphthalene, 1-methylnaphthalene, and2-methylnaphthalene are labeled. FIG. 11 (bottom) shows the absorbancespectrum for the naphthalene peak, where the maximal response is indeedin the 200 nm-220 nm region.

In practice, any number of filters can be applied either during a GC/VUVrun or afterward. A run-time chromatogram plot might overlay the resultsof several filters, each designed to enhance the response for aparticular class of analyte.

While a particular advantage of the disclosed techniques is the abilityto perform quantitative analysis without having to be “taught” what aparticular amount of an analyte looks like using a set of calibrationsamples, it is still possible to use the VUV spectroscopy system in thismanner. A VUV detector response can be correlated with known amounts ofanalyte injected directly into the detector or into a GC separationprocess to later be detected by the VUV spectroscopy instrument. Many GCdetectors are already used in this manner, and the enhanced or moreuniversal response provided by the VUV detector may be more than enoughto justify its placement within such measurement processes, even if itsfull three-dimensional data characteristics go underutilized.

For example, several samples each having a known amount of an analyte ofinterest would be injected into a GC/VUV system and a response measured.The samples would be constructed to include analyte amounts that span atypical measurement range (or larger). The VUV detector response couldbe a normalized integrated absorbance over a particular wavelengthregion as in Eqn. 23 or variations therein. Some aspect of the responsewould be plotted against the known amount of analyte for each sample.For example, the maximum response for the analyte, taken from thelargest value of the associated chromatogram peak, may be correlatedwith the known injected amounts. Alternately, the chromatogramassociated with the analyte may be further integrated, and an area ofthe peak under the response versus time curve correlated with the knownanalyte amounts. Once a response has been obtained for all of thecalibration samples, a calibration curve consisting of analyte amountversus response is generated. The curve is ideally linear, but otherfunctional relationships are possible. The detector response for asample containing an unknown amount of the analyte can then be measured,and from the functional relationship between the detector response andanalyte amount generated during the calibration process, the unknownanalyte amount determined.

Another example of a chromatogram generated using techniques disclosedherein is presented in FIG. 12. The chromatogram corresponds to a simplesolution of two common solvents; namely methanol and methylene chloride.The detector was configured to collect a scan every 200 ms. A normalizedintegrated transmittance value was constructed for each scan by summingthe transmittance over the 125-180 nm wavelength region and dividing bythe total number of data points (Eqn. 23 applied to transmittance). Asevident in the figure, two well-defined peaks corresponding to (I)methanol and (II) methylene chloride are apparent at ˜31 and ˜38 s,respectively. While this assignment is readily made upon considerationof the well established elution times for methanol and methylenechloride, the current system does provide a powerful means of verifyingthis conclusion.

The transmission spectra associated with the methanol and methylenechloride peaks of the previous figure are presented in FIGS. 13 and 14respectively. As is evident, the two spectra are clearly distinguishableto the naked eye. The methanol transmission spectrum of FIG. 13 exhibitscharacteristic absorption doublets at ˜148 nm and ˜158 nm, while themethylene chloride spectrum of FIG. 14 shows features near ˜138 nm and˜152 nm. With knowledge of the VUV absorption cross sections for thesetwo species, the concentrations of each can be readily determined usinga computerized linear regression algorithm as previously described. Theanalysis may be performed in real-time (i.e. during collection of thechromatograph) or post measurement. Furthermore, the analysis may beperformed in situations where a single analyte is present in the flowcell (as in FIGS. 13 and 14) or in cases where multiple componentsco-elute.

VUV absorption cross-sections for species of interest can be found inthe literature or determined through measurement using the methodsdescribed above. While cross sections could be determined during normaloperation (i.e. where analytes are introduced via the GC column), insome cases it may be advantageous to isolate the species of interest inthe gas cell for an extended period of time so as to facilitate signalaveraging, thereby reducing uncertainty. This approach could be used fora variety of reasons including, but not limited to, trace analysis andimproved accuracy during cross-section determination.

Isolation may be accomplished in a variety of means including, but notlimited to, reducing or altogether stopping make-up gas flow, use ofappropriate three way valves at the inlet and outlet of the flow cell,or by modifying the cell so as to facilitate direct injection ofsamples. The latter could be achieved using the flow cells previouslydescribed, or instead with a stand alone version of the same which wouldnot require the use of an accompanying GC.

An example of such a system is presented in FIG. 15 which depicts a VUVdetector equipped with a stand alone cell 1500. Other source anddetector module features are similar to that of the system of FIG. 1 asmay be seen from the figures. The cell is outfitted with the requiredinjection, pumping and backfill ports 1502, 1504 and 1506. While notexplicitly shown in the figure the cell is also equipped with a means ofheating. Just as described earlier, it may be desirable to coat and/ortreat the inside of the cell in such a manner as to render it inert toreactive species. Since multiple species can be introduced to the cellin a controlled manner the system could also be used to study chemicalreactions and the like. It is noted that the cell may also be equippedwith other accessories (i.e. light sources, probes, sensors, electrodes,etc) so as to further enhance this ability.

Liquid chromatography (LC) is similar in many respects to gaschromatography. In LC the sample is transported with a liquid solvent(referred to as the mobile phase) along a column. The column consists ofa stationary phase that interacts with the various components of thesample. The interaction of the sample components with the stationaryphase causes them to elute from the end of the column at differenttimes, with the result that the sample is “separated” into itsconstituent components. Eluted components are again detected by means ofa detector. Modern day LC systems generally utilize very small particlesin the stationary phase and relatively high pressures and are thus arereferred to as high performance liquid chromatography (HPLC) systems.

The most common HPLC detector is the UV-Vis (ultraviolet-visible)absorption detector. In principal an absorption detector extending downinto the VUV should prove orders of magnitude more sensitive as a resultof the much higher absorption cross sections exhibited by most moleculesin the VUV (relative to that of the UV-Vis region). Unfortunately, thepotential benefits of this approach have yet proved unattainable usingstandard bench-top systems since the higher cross sections rendermacroscopic thicknesses of all liquids virtually opaque in the VUV. As aresult, VUV absorption investigations of liquids have been almostentirely limited to systems coupled to dedicated VUV beam lines atmassive synchrotron radiation facilities, where incredibly intense lightsources are available.

It follows that there would be tremendous benefit from development of abench top VUV absorption system that could operate using conventionalsources. To achieve this end in one embodiment the current systemincorporates an ultra short path length sample cell so as to render thinfilms of liquid semi-transparent to VUV light.

A side view of one embodiment of the short path length sample cell isdepicted in FIG. 16. Three general regions are evident in the figure; atop barrier region 1602, a bottom barrier region 1604 and a centralchannel region 1606. On the right and left hand sides of the centralchannel region, inlet and outlet ports 1610 and 1612 can be discerned.The dotted circle 1620 in the center of the channel region representsthe area where the VUV light beam passes through the ultra short pathlength flow cell. In operation liquid from the HPLC enters the flow cellthrough the inlet port and spreads throughout the channel region as itflows across the area sampled by the VUV light beam. The liquidcontinues on and exits the cell via the outlet port.

The cell is constructed of two VUV transparent windows that aresandwiched together. One of the windows is modified such that apatterned ultra thin film is present on one side. Areas where the filmis present form the barrier regions; while those without film form thecentral channel region. The cell is designed so as to maintain crosssectional area along the flow direction axis; thus ensuring laminarflow. To achieve this end the profiles of the area sampled by the VUVlight beam and the remainder of the central channel region are quitedistinct as evident in FIG. 17.

The three drawings of FIG. 17 depict cross sectional representations ofthe flow cell taken along the flow direction axis and corresponding tothe position of the vertical dashed lines 1650, 1652, and 1654 of FIG.16. Specifically, the top schematic 1702 in FIG. 17 corresponds to thedashed line 1650 nearest the center of the flow cell in FIG. 16. In thisregion of the cell the profile consists simply of a thin horizontalchannel 1710 through which the liquid will flow. The walls of thechannel are formed by the patterned thin film deposited on the bottomwindow 1712. The top and bottom windows 1714 and 1712 form the top andbottom of the channel respectively.

Similarly, the middle schematic 1704 of FIG. 17 corresponds to themiddle dashed line 1652 in FIG. 16. In this drawing a third region 1720is evident between the flow channel and barrier region on either side ofthe schematic. This region is created by removing material from thebottom window via an etching process or the like. It serves the purposeof maintaining the cross sectional area so as to promote laminar flow ofliquid through the cell. While the walls of the etched region aredepicted as vertical in the figure, it is understood that othergeometries will be better suited, and hence employed, to enhance laminarflow. It is further noted that the cell is designed such that the VUVlight beam only passes through the un-etched portion of the flow regionso as to avoid scattering losses due to post-etching roughness. It isnoted that in some instances it may be desirable to extend the etchedregion along the entire length of flow channel so as not to impedecolumn flow.

Finally, the bottom drawing 1706 of FIG. 17 corresponds to theright-most dashed line 1654 of FIG. 16. This portion of the flow cellcontains etched region 1730 and does not contain an un-etched portion ofthe channel since light does not pass through this portion of the cell.As is evident the greater etch depth (relative to the middle drawing) isnecessary to maintain the cross sectional area in light of the reducedwidth of the channel. Again, it is understood that more complicatedprofiles would be employed to reduce “dead” volume and enhance laminarflow.

FIG. 18 presents an embodiment of a VUV HPLC detector 1800 incorporatingthe ultra thin path length flow cell. In fact, this configuration isquite similar to the VUV GC detector of FIG. 1; wherein the gas flowcell has been replaced by the ultra thin path length liquid flow cell1802. As before, the sample exiting the chromatography system (in thiscase the HPLC system) will enter the cell at the inlet port 1806 as itexits the column 1804 and interacts with the collimated VUV light beam.The liquid leaving the cell does so through the exit port 1808. Whilenot explicitly shown in the figure it is understood that the system maybe equipped with other accessories like make-up solvent fittings,heaters, coolers, and the like.

Yet another embodiment of the concepts disclosed herein is presented inFIG. 19 which depicts a VUV HPLC detector 1900 employing a focused beamand smaller liquid flow cell 1902. Again, this system is somewhatanalogous to the focused beam VUV GC detector system presented earlierin FIG. 4. This configuration offers the promise of higher photon fluxthan the collimated version of FIG. 18 and may be useful in instanceswhere strongly absorbing species are concerned.

While the embodiments described above in FIGS. 16-19 permit liquids tobe directly studied using VUV absorption techniques, it is generallytrue that the VUV absorption spectra of gases are richer in structurethan those of liquids and as such, considerably more useful inidentification applications. Unfortunately many analytes, particularlylarge, fragile molecules of biological interest, are insufficientlyvolatile or thermally stable to withstand analysis using GC separationtechniques.

Electrospray ionization is a means of generating very fine liquidaerosols through use of electrostatic charging. In fact, the techniquehas become a standard means of producing intact ions in vacuum fromlarge and complex species in solution for study using mass spectrometricanalysis. In the electrospray process a solution of analyte is passedthrough a capillary held at high potential. The effect of the highelectric field as the solution emerges is to generate a mist of highlycharged droplets. Nebulization of the solution emerging from thecapillary may be further facilitated by the flow of a nebulizer sheathgas. The emerging droplets pass through a potential and pressuregradient towards the analyzer portion of the detector. During theirtravel, the droplets reduce in size through evaporation of the solventand droplet subdivision. Ultimately, fully desolvated ions result fromcomplete evaporation of the solvent or by field desorption from thecharged droplets. To hasten the evaporation of the solvent a heatedcountercurrent flow of dry gas is often added.

While the preparation requirements of analytes for analysis using MS andVUV gas absorption methods are fundamentally different (MS requires thecreation of charged ions while VUV gas absorption does not), they bothrequire that molecules be rendered in gaseous form before they can beintroduced for detection. It follows that with appropriatemodifications, electrospray techniques may also lend themselves to usein converting, without damage, heavy molecule liquid samples (from anHPLC system or otherwise) into gaseous species for study using a VUV GCdetector such as described earlier in this disclosure.

Another embodiment of the disclosed systems is presented in FIG. 20which depicts an electrospray interface integrated into a VUV gasabsorption flow cell. Liquid sample 2002 enters the flow cell through anelectrospray capillary 2004 which is held at high potential relative tosurrounding electrodes (not explicitly shown) which help shape thedistribution of potential and the flow of any heated dry gas that may beused. The resulting field at the tip of the needle charges the surfaceof the emerging liquid, dispersing it by Coulomb forces into a finespray of charged droplets. Driven by the electric field, the dropletsmigrate across a pressure/potential gradient toward the sampling volumeof the VUV light beam 2010 which passes through the two VUV transparentwindows 2012.

The density and distribution of fully desolvated analyte molecules inthe sampling volume is influenced by many factors including the liquidsample flow rate, nature of the sample (both analyte and solvent),capillary potential, electrode details (location, geometry, potential,etc.), properties of drying gas (type, temperature, flow rate, etc),flow cell environment and flow cell geometry. While just two pairs ofsimple electrodes 2020 are displayed in the figure, it is understoodthat additional more complicated electrodes, (with positive and/ornegative potential relative to the capillary) could be employed in orderto ensure an optimum distribution of analyte molecules is maintained inthe sampling volume.

It follows that this flow cell could readily be incorporated into any ofthe VUV gas absorption systems previously presented in this disclosure.

It is further noted that any of the gas or liquid flow cells describedin this disclosure could also be coupled with a VUV circular dichroismspectrometer, for example as described in co-pending U.S. patentapplication Ser. No. 13/184,619 filed on Jul. 18, 2011, the contents ofwhich are expressly incorporated herein by reference. This configurationcould prove particular advantageous in applications involvingstereoisomers.

As described above a wide variety of detection hardware and techniqueshave been provided. It will be recognized that the various conceptsdescribed herein may be utilized singularly or alternatively in variouscombinations.

In one aspect, a gas flow cell is described herein. In certainembodiments, the gas flow cell may be configured to provide a gasdetection space having volume equal to or greater than the analytevolume so as to allow substantially all of the analyte molecules to becontained within the flow cell simultaneously. By providing more analytein the flow cell, absorption may be maximized to yield a more sensitivedetector system. In one embodiment, the volume is provided such that itequals or exceeds the analyte volume provided from a gas chromatographycolumn. It will be recognized, however, that other lesser volumes of thegas flow cell may be utilized while still obtaining at least some of thebenefits described herein.

In another aspect, a gas flow cell that is coupled to a make-up gas flowis provided. The make-up gas may be a comprised of a gas that isrelatively invisible to the detection scheme. In this manner the gasflow level of the make-up gas may be adjusted without substantiallyimpacting system sensitivity. In one embodiment, analytes are providedto the gas flow cell in a temporal manner. The make-up gas flow may beadjusted to maintain the temporal resolution of the analytes, yetbecause the detection system is relatively insensitive to the make-upgas, the adjustment of the gas flow will not otherwise reduce systemsensitivity. Such a system is particularly well suited for use with agas chromatography column which may produce multiple analytes separatedover time.

In yet another aspect the gas flow cell may be equipped with thermalcouplings so as to provide thermal isolation from other system modulesto which the gas flow cell may be coupled. The gas flow cell may also besealed to provide a controlled environment within which the gas beingdetected is present. In still yet another aspect, the gas flow cell maybe comprised to accept collimated light. Further, the gas flow cell maybe dimensioned so as to allow access of sufficient collimated lightenergy so that an absorption measurement may be obtained. Alternatively,focusing optics may be provided so that a focused light beam of smallcross-sectional area may be utilized in the absorption detection scheme.Depending upon the application, it may also be desirable to provide amechanism to heat the gas flow cell.

It will be recognized that the various features of the gas flow cellsdescribed above may be utilized separately while still obtainingbenefits as described herein. Furthermore, the gas flow cells describedherein may be utilized within a larger detection system in a variety ofmanners, again using a variety of the features described above eithersingularly or in combination. In one embodiment, the gas flow cell maybe directly coupled to other system modules in a manner thataccommodates thermal expansion of the flow cell. In such an embodiment,other system modules connected to the flow cell may be configured tomove as the flow cell expands due to thermal effects. In anotherembodiment, the gas flow cell may be isolated from the other modules ofthe system such that thermal expansion of the gas flow does not requiremovement of other modules. In one such embodiment, the gas flow cell maybe contained with a separate dedicated chamber. In one embodiment of theimplementation of such chamber, both the gas flow cell and the dedicatedchamber may have sealed controlled environments, the environmentshowever being optically coupled to each other, for example through opticwindows.

One exemplary embodiment of the detection systems for which thedisclosed techniques may be utilized is a detection system thatcomprises a gas chromatography column. One exemplary embodiment of thedetection systems in which the disclosed techniques may be utilized is avacuum ultra-violet (VUV) optical spectroscopy system. Generally, VUVlight is considered to be wavelengths of light of about 190 nm and less.In one exemplary embodiment, a VUV light source may be utilized toanalyze the output of a gas chromatography column by analyzing the VUVoutput of a flow cell through the use of spectrometer detection module.In one embodiment the VUV light source may be a broad-band VUV lightsource that exposes the analyte in the flow cell to multiple lightwavelengths simultaneously.

Though described above with regard to a spectrometer detector module, agas chromatography analyte source and a VUV source module, it will berecognized that each of these components may be replaced with othermodules or components. Thus for example, non-gas chromatography analytesources may be utilized, non-VUV light wavelengths may be utilized andnon-spectrometer detector modules may be utilized while still obtainingone or more of the benefits described herein.

One non gas chromatography analyte source may be an electrospray analytesource. In such a technique a liquid source may be provided to anelectrospray capillary that is maintained at a high potential. Theoutput of the electrospray capillary may be provided to a detectionchamber, such as an environmentally controlled chamber that includes alight path (for example a VUV light path). Electrodes at the exit of thecapillary may be utilized to aid in the desired distribution of theanalyte in the sampling volume. A VUV transparent make-up gas may alsobe used to aid the generation of the desolvated analyte molecules in thesample volume.

In yet another embodiment, an ultra-short path length liquid flow cellmay be provided. The liquid flow cell may be formed through the use oftwo VUV transparent windows. A thin film may be formed on one or more ofthe windows and then removed in regions to clear optical regions throughwhich fluid may be provided through inlet and outlet ports. In thismanner, the thickness of the clear optical region may be defined by thethickness of the thin film. The fluid conduction regions may be formedso as to maintain a desired cross section area and promote laminate flowthrough the cell. The liquid flow cell may be used in conjunction with aliquid chromatography system, VUV light source and/or spectrometerdetection systems. In some embodiments liquid flow cell may rigidlyconnected to the other system components or may be contained in aseparate dedicated flow cell chamber. Collimated light or focused lightmay be passed through the liquid flow cell.

The techniques described above support a wide range of methods ofanalyzing materials. These methods may be utilized independently or invarious combinations and the disclosure provided herein is not meant tobe limited to any particular analysis method. In one embodiment adetector for gas chromatography applications is described. In anotherembodiment a spectroscopy detector for gas chromatography (GC)applications utilizing vacuum ultra-violet (VUV) wavelengths isdescribed. The GC application utilizing VUV wavelengths, wavelengths atwhich most materials exhibit much stronger and richer absorptioncharacteristics than at, e.g., ultra-violet and visible wavelengths,provides enhanced sensitivity to analytes separated during the GCprocess. Utilizing a spectroscopy detector and VUV wavelengths for GCapplications yields a three-dimensional dataset that enables bothquantitative and qualitative capabilities. This three-dimensionaldataset may include absorption data, wavelength data and time data. Thedata can be fit to determine amounts of eluting analytes, compared withknown analyte spectra to identify eluting components, or fit against amodel consisting of multiple analytes to determine amounts of coelutingspecies. Two-dimensional responses can be generated by applying spectralfilters that integrate absorbance/transmittance data over specificwavelength regions, enhancing chromatogram responses to particularclasses of analytes. A system using these techniques is not adverselyaffected by the relative amount of carrier/makeup gas to amount ofanalyte, and thus benefits from utilizing a variable and controllablemakeup gas flow to the detector cell. The makeup gas flow can beincreased in order to enhance the time-resolution of GC/VUVchromatograms, decreased to improve measurement statistics, or optimizedto achieve both to the extent possible.

In another embodiment, a method for determining an unknown cross sectionvalue of an analyte at one or more wavelengths is provided. The crosssection of the analyte provides the absorption profile of the analyte asa function of wavelength. The method may include performing a separationof a sample comprising known amounts of analyte molecules and solventmolecules where the absorption cross section of the analyte is unknownand the absorption cross section of the solvent molecules is known. Themethod further includes forming a ratio of measured analyte and solventabsorbances. The method further includes computing from the absorbanceratio the cross section of the unknown analyte by making use of (1) theratio of the known amounts of the analyte and the solvent and (2) theknown cross section of the solvent. Since the absolute amounts ofanalyte and solvent are not invoked, the method is impervious tovariations in injection volume or other systematic errors that affectanalyte and solvent indiscriminately.

In another embodiment, a method for determining the wavelength-dependentcross section for an unknown analyte is provided. The method may includemeasuring the absorbance of a sample that consists of only the analytein question. This may be achieved by either direct injection of a puresample, or by utilizing a GC separation process. The method may furtherinclude forming a relative absorbance by normalizing the spectral datausing the absorbance value at a single wavelength. This relativeabsorbance is equal to the relative cross section of the analyte,regardless of the number of analyte molecules present during themeasurement. The method then includes calculating the absolute crosssection for the entire wavelength region by making use of the absolutecross section at a single wavelength. Most simply, the normalizationwavelength corresponds to this known wavelength.

Yet another method provides for determining the wavelength-dependentcross section for an unknown analyte. This method may include performinga separation of a sample comprising known amounts of analyte moleculeshaving an unknown cross section and known amounts of solvent moleculeshaving a known cross section. GC and makeup gas parameters may beadjusted to enhance the absorbance characteristics of analyte andsolvent at a single wavelength, or possibly a small number ofwavelengths. A ratio may then be formed of the measured absorbances ofthe analyte and solvent. The method may then further include computingthe cross section for the analyte at the single or small number ofwavelengths from the absorbance ratio, making use of the ratio of theknown amounts of the analyte and solvent, and the known cross section ofthe solvent. A further extension of the method may include performing asecond absorbance measurement of the analyte. The second measurementneed not involve the first sample, nor the same GC and makeup gassettings. The GC and makeup gas settings may instead be optimizedstrictly for enhancement of the analyte absorbance signal. The methodthen includes forming a relative absorbance spectrum from the secondmeasurement by normalizing with the absorbance value at one wavelengthand then calculating the absolute cross section for the entirewavelength region from the relative absorbance spectrum, where the knowncross section value is the one determined using the absorbance ratio.

Another method disclosed is a method for identifying analyte componentsin a measured GC/VUV chromatogram. The method includes storing crosssections for known/previously measured analytes in a library database.Then a relative absorbance is constructed by normalizing each absorbancespectrum in a chromatogram, or within a specific time region of thechromatogram, by the absorbance at a particular wavelength. The methodfurther includes comparing the relative absorbance with relative crosssections obtained by normalizing the cross section spectra in thedatabase. Preferably the relative cross sections are generated using thesame wavelength as was used to construct the relative absorbancespectra.

Yet another method disclosed is a method for identifying/measuringanalyte components in a GC/VUV chromatogram. The method includes storingcross sections for known/previously measured analytes in a librarydatabase. The method further includes electing a number of candidateanalytes according to possible presence in the measured sample. Themethod also includes refining the number of candidate analytes relevantwithin a particular time window on the chromatogram according toestimated analyte retention times and performing a regression/fitprocedure that optimizes the set of analyte amounts (in absolute numberof molecules or concentration), using absorbance spectra within the timewindow identified in the previous step.

A method for utilizing a known sample for monitoring GC efficiency isalso disclosed. GC detectors are typically calibrated using a set ofknown samples with analyte concentrations that span expected measurementranges. Such calibrations account for variations in detector response toanalyte concentration, as well systematic errors in the GC injection andseparation process. These can include variations in the transfer ofsample from injector to column, errors in split flow or split ratio,losses due to leaks, column efficiency, etc. These variations can bedifferent for different GCs and can vary with time for a single GC. Asdisclosed herein the VUV detector response with respect to the amount ofmeasured analyte does not need to be calibrated by use of an externalstandard: the measured transmittance or absorbance is always the samefor a given amount of a specific analyte within a given sample cellgeometry. In addition, with a known analyte cross section, the VUVdetector can determine the amount of injected analyte that actuallyreaches the detector, without having to know anything about theefficiency of the GC transfer/separation process.

In one embodiment, a sample may be prepared and characterized bymeasuring the amount of analyte that reaches the VUV detector after a GCseparation process. The same sample can be measured again later, and anydifferences in the amount of analyte measured attributed to differencesin the aforementioned variations in the GC process. Alternately, thesample can be measured on two GC/VUV systems, and efficiency of the GCscompared by comparing the amount of analyte reaching the detector ineach case.

Further modifications and alternative embodiments of this invention willbe apparent to those skilled in the art in view of this description.Accordingly, this description is to be construed as illustrative onlyand is for the purpose of teaching those skilled in the art the mannerof carrying out the invention. It is to be understood that the forms ofthe invention herein shown and described are to be taken as presentlypreferred embodiments. Equivalent elements may be substituted for thoseillustrated and describe herein and certain features of the inventionmay be utilized independently of the use of other features, all as wouldbe apparent to one skilled in the art after having the benefit of thisdescription of the invention.

What is claimed is:
 1. A method of obtaining data regarding a gasanalyte, comprising: providing the analyte from a gas chromatographycolumn; providing the analyte to a gas flow cell; passing VUVwavelengths or less of light through the gas flow cell; detectingabsorption of the VUV wavelengths or less of light by the analytethrough the use of a spectrometer optically coupled to an optical outputof the gas flow cell; obtaining a three dimensional data set indicativeof an amount of absorption of the analyte as a function of wavelengthand time; and analyzing the three dimensional data set to determine anidentity and quantity of the analyte.
 2. The method of claim 1, furthercomprising providing a makeup gas flow to the gas flow cell inconjunction with the analyte.
 3. The method of claim 2, furthercomprising varying the flow rate of the makeup gas flow.
 4. The methodof claim 2, further comprising varying the flow rate of the makeup gasflow in a manner to enhance temporal resolution of the analyte providedfrom the gas chromatography column in a manner that does not affect thedetection of the absorption of the analyte.
 5. The method of claim 2,further comprising varying the flow rate of the makeup gas flow in amanner to ensure substantially all analyte molecules are present in thegas flow cell during detection.
 6. The method of claim 2, furthercomprising varying the flow rate of the makeup gas flow in a manner toimprove absorption measurements.
 7. The method of claim 1, wherein theVUV light source is a broadband light source and the absorption ofmultiple wavelengths of light is detected simultaneously.
 8. The methodof claim 1, wherein a flow channel is provided in the gas flow cell, theflow channel being in the light path of the gas flow cell, the volume ofthe flow channel being approximately equal to or greater than a volumeof the analyte provided from the gas chromatography column.
 9. Themethod of claim 1, wherein two or more analytes are provided by the gaschromatography column.
 10. The method of claim 9, wherein at least twoanalytes are provided simultaneously.
 11. The method of claim 1, whereinthe analysis of the three dimensional data set comprises at least anoptimization procedure.
 12. The method of claim 11, wherein theoptimization procedure comprises constructing a model for an amount ofabsorption of the analyte and comparing it to the detected absorption.13. The method of claim 12, wherein the optimization procedure furthercomprises iteratively adjusting inputs to the model in an effort tominimize differences between the modeled and detected absorption.
 14. Amethod of obtaining data regarding a gas analyte, comprising: providingthe analyte from a gas chromatography column; providing the analyte to agas flow cell; allowing for movement of the gas flow cell from thermalexpansion of the gas flow cell by allowing for movement of at least oneof a light source coupled to the gas flow cell or a detector coupled tothe gas flow cell; passing collimated VUV wavelengths or less of lightthrough the gas flow cell; detecting absorption of the VUV wavelengthsor less of light by the analyte through the use of a spectrometeroptically coupled to an optical output of the gas flow cell; obtaining athree dimensional data set indicative of an amount of absorption of theanalyte as a function of wavelength and time; and analyzing the threedimensional data set to determine an identity and quantity of theanalyte.
 15. The method of claim 14, further comprising providing amakeup gas flow to the gas flow cell in conjunction with the analyte.16. The method of claim 15, further comprising varying the flow rate ofthe makeup gas flow.
 17. The method of claim 15, further comprisingvarying the flow rate of the makeup gas flow in a manner to enhancetemporal resolution of the analyte provided from the gas chromatographycolumn in a manner that does not affect the detection of the absorptionof the analyte.
 18. The method of claim 15, further comprising varyingthe flow rate of the makeup gas flow in a manner to ensure substantiallyall analyte molecules are present in the gas flow cell during detection.19. The method of claim 15, further comprising varying the flow rate ofthe makeup gas flow in a manner to improve absorption measurements. 20.The method of claim 14, wherein the VUV light source is a broadbandlight source and the absorption of multiple wavelengths of light isdetected simultaneously.
 21. The method of claim 14, wherein a flowchannel is provided in the gas flow cell, the flow channel being in thelight path of the gas flow cell, the volume of the flow channel beingapproximately equal to or greater than a volume of the analyte providedfrom the gas chromatography column.
 22. The method of claim 14, whereintwo or more analytes are provided by the gas chromatography column. 23.The method of claim 22, wherein at least two analytes are providedsimultaneously.
 24. The method of claim 14, wherein the analysis of thethree dimensional data set comprises at least an optimization procedure.25. The method of claim 24, wherein the optimization procedure comprisesconstructing a model for an amount of absorption of the analyte andcomparing it to the detected absorption.
 26. The method of claim 25,wherein the optimization procedure further comprises iterativelyadjusting inputs to the model in an effort to minimize differencesbetween the modeled and detected absorption.